In facsimile equipment, for example, a stepping motor is used as a driving source for an original document scanning mechanism and/or a recording paper conveying mechanism.
FIG. 1 shows a conventional exciting circuit for a stepping motor used as such a driving source. In this circuit, one end of each exciting coils 1 and 2 for two phases of a stepping motor is connected commonly through a resistor 3 to a power source line 4. The other ends of the exciting coils 1 and 2 are grounded through collector-emitter circuits of switching transistors 5 and 6, respectively. Similarly, one end of each of the exciting coils 7 and 8 for the other two phases of the stepping motor are connected commonly through a resistor 9 to the power source line 4, and the other ends of coils 7 and 8 are grounded through collector-emitter circuits of switching transistors 11 and 12, respectively. The respective bases of the switching transistors 5, 6, 11 and 12 are connected to the four output terminals of a decoder 13. The decoder 13 serves to selectively turn on the switching transistors 5, 6, 11 and 12 in accordance with an output signal of a counter. The counter counts a motor clock pulse 14 so as to control the excitation of the exciting coils 1, 2, 7 and 8, thereby driving the stepping motor.
When the stepping motor is intermittently driven with this exciting circuit, a stepwise displacement characteristic is exhibited in a low speed driving period, as shown in FIG. 2. That is, the rotary angle of the motor is displaced in a stepwise manner each time the motor clock pulse is produced. On the other hand, if the motor is driven at a high speed in the range of 5-10 milliseconds per step, the rise in displacement in response to the motor clock pulse 14 becomes relative. Since the motor shaft stops for only a very short time during each stoppage period, poor performance in its picture image reading/recording function results.
To solve such a problem which occurs during a high speed driving period, there has been proposed a countermeasure in which the resistors 3 and 9 disposed between the power line 4 and each of the exciting coils 1, 2, 7 and 8, respectively, are replaced by those resistors which have high resistance values. If the resistors 3 and 9 are replaced by resistors having high resistance values, the electrical time constants constituted by the respective inductance values of the exciting coils 1, 2, 7 and 8 are reduced. As such, quick rise times in the excitation currents are preserved (and even enhanced). However, in the case where a stepping motor is driven by such an exciting system as mentioned above, the power loss increases in accordance with the increments of the resistance values, which result in the reduction of driving efficiency relative to the power source for the stepping motor. Further, the amount of heat generated by the resistors 3 and 9 increases in accordance with the reduction of the driving efficiency. This higher amount of heat generation may adversely affect the circuit elements surrounding these resistors. Further, during a linear driving period in which no stepwise displacement is necessary (for example, in the case of white line skipping), noise is generated in the driving system by the displacement characteristic as shown in FIG. 3.